Access the weight values of a polygon
OP (me) didn't realise that a weight value of 0 auto-removes a vertex from the group. Also you need to use the index for accessing the weight data. So this works:

import bpy

ob = bpy.context.active_object
vertexGrp = ob.vertex_groups["Group"].index
for poly in ob.data.polygons:
    weight = 0    
    try:                    
        for vert in poly.vertices:           
            weight += ob.data.vertices[vert].groups[vertexGrp].weight
        weight /= len(poly.vertices)
        print(weight)
    except:
        print("no vgroup")